This invention relates to an improved rotary drum comminutor installation.
A comminutor is a device designed to cut or break up large solids present in a fluid stream without having to remove the solids from the liquid. Comminutors are typically employed in waste treatment plants where they can improve efficiency and lower operation and maintenance costs. In smaller treatment plants, comminution can significantly improve efficiency by reducing the size of incoming solids and allowing more complete biological reduction of organic materials. In larger treatment systems, comminution can significantly improve performance of downstream equipment. Comminution can also reduce clogging of pipes and small pumps, such as foam control systems, reducing maintenance and labor costs.
A rotary drum comminutor is a type of comminutor having a generally cylindrical slotted drum which is rotatively driven about an upright axis and in which the fluid travels axially downward through the comminutor after it passes through the slots in the drum.
In a typical rotary drum comminutor installation, the comminutor is mounted in a spiral-shaped concrete chamber, with the base of the comminutor being depressed below the level of the incoming sewage. Sewage is delivered to the inlet end of the chamber and is distributed around the cylindrical drum of the comminutor by the spiral-shaped flow passage. When the fluid reaches the comminutor, the liquid and very small solid particles pass through the drum slots into the hollow interior, down through the bottom of the drum, through an inverted siphon formed in the base of the concrete chamber, and into the downstream effluent channel. Solids that are too large to pass through the drum slots are held against the drum by the pressure of the flow and are carried by the rotation of the drum to stationary combs. Cutting teeth attached to the drum cut the solids held against the combs into particles small enough to pass through the drum slots. The spiral flow of the liquid in the spiral-shaped chamber returns any partially cut solids to the drum to repeat the cutting process until the particles are sufficiently small to pass through the drum slots.
One of the major drawbacks of rotary drum comminutors, particularly in large comminutor sizes, is the complicated concrete chamber required to create the spiral-shaped passage surrounding the comminutor. Special concrete forms are required to pour this chamber, which usually must be shipped to the installation site by the comminutor manufacturer, and additional time, materials and expense are required for site preparation and fabrication of the chamber. In addition, rotary drum comminutors normally have bases which cause substantial fluid head loss. Loss of head is an important factor in the cost of construction and operation of a sewage treatment facility and is a significant drawback to the use of rotary drum comminutors. While some of this head loss can be corrected by construction of concrete return bends and other design features, such features increase the cost and complication of the installation. Moreover, the aforementioned design features of a normal rotary drum comminutor require a concrete chamber bottom of considerable structural strength. The concrete thickness required to achieve this strength creates an additional head loss and adds to the cost of construction and operation of the comminutor installation.